- 发布时间：2021-05-11 20:45
- 发布时间：2021-05-11 20:45
Demonstration of the best efficiency method for improving the injection molding plastic pallet machine
In this increasingly competitive business society, improving consumption efficiency is an issue worthy of attention. After this article, you will understand the ways to improve the working efficiency of the injection molding machine, and at the same time be able to fully understand the working principle of the injection molding machine and the knowledge of injection molding.
Injection plastic pallet cycle
The injection cycle of a hydraulically driven injection molding machine is from the beginning of mold clamping to the next mold clamping.
Mold clamping is generally divided into four sections: fast mold clamping, slow mold clamping, low pressure mold protection and high pressure mold clamping. The injection starts after the high-pressure clamping is completed, and is also divided into multiple sections. The molten plastic fills the cavity during injection. When the cavity is filled, the pressure rises sharply, so the end of the injection is also called the packing phase (packing phase). When the control is inappropriate, the waste will produce burrs.
The pressure holding starts after the injection is completed. In fact, the cooling starts after the cavity is filled and overflowed, that is, from the pressure holding. When the mold cools, the waste product shrinks due to cold. The function of holding pressure is to pass through the cold runner that has not yet solidified, and fill the recesses formed by the shrinkage at a holding pressure that is generally lower than the injection pressure, so that the waste products are full (without dents) when they are demolded.
When the cold runner is solidified, it is meaningless to hold the pressure again, and the pressure can be terminated. The holding pressure can be divided into multiple sections, and the holding pressure of each section is different (usually it is gradually increased section by section), divided by time. The total holding time is determined by weighing the weight of the waste product or there is no dent from the waste product. Start with a short holding time, and increase the holding time for each injection, until the weight of the waste no longer increases or the dents can be tolerated, the holding time does not need to be increased.
The "cooling time" parameter set on the injection molding machine is a period of time from the completion of the pressure holding to the opening of the mold, but the cooling begins as early as the mold cavity is filled with plastic. The purpose of "cooling time" is to make the waste product continue to cool and solidify so that it will not be deformed due to ejection when it is ejected. It should be said that the "cooling time" is derived from experiments.
At the beginning of the "cooling time", the feeding stops at the same time. It appears that the "cooling time" is longer than the feeding time. It is also possible that the feeding time is longer than the "cooling time". In other words, the plasticization ability of the screw is lacking and it becomes a bottleneck. Therefore, increasing the plasticization ability is the way to shorten the cycle time in this case.
"Cooling time or "After the filling is completed, the mold will be opened, the waste product will be ejected once or repeatedly, and the mold will be closed again after the ejection, and the next cycle will begin immediately.
Recently designed toggle-type injection molding machines have regenerated mold clamping oil circuits to strive for higher mold clamping speed. It is suitable for use under the premise that the mold is not subject to high impact.
high pressure mold clamping
Using the lowest clamping force that can prevent the waste from producing burrs, which can shorten the time required for the high-pressure clamping section. Moreover, the tie rods, toggles, and templates of molds, injection molding machines, and templates will also have a short life due to the use of low clamping force. If a mold with a clamping force of 50t is sufficient, there is no need to use a clamping force of 51t, although your injection molding machine has a higher clamping force.
The highest injection speed can be used under the condition that the waste does not produce bubbles or black spots due to scorched plastic. Especially for thick-walled injection molding, a large amount of air-storing space in the mold cavity is filled with molten plastic. Too high injection speed prevents the air in the cavity from being discharged out of the mold, resulting in bubbles.
Using the lowest injection pressure can correspondingly reduce the required clamping force (expansion force), while using the lowest barrel temperature can shorten the "cooling time".
The shortest holding time can be determined from the weight of the waste product or the acceptable dent. There are many thin-walled products that do not need to hold pressure, because the inner layer of the waste product is basically solidified at the end of the injection.
I heard a saying: The mold is basically a heat exchanger. Yes, the mold will take away the heat of melting from time to time through the cold water channel, and a properly designed mold can improve the efficiency of heat exchange.
However, under permitted conditions, ice water cooling can shorten the "cooling time". If the ice water cooling makes the mold condensation, the dry fan and the sealed mold clamping installation can reduce the dew point and avoid condensation.
If the plasticization ability is not enough and it becomes a bottleneck, the following measures can be taken during screw design and parameter adjustment:
1. Barrier screw can increase plasticizing ability.
2. Large diameter (C) screw can increase plasticizing ability.
3. Increasing the groove depth of the screw can increase the plasticizing ability.
4. Increasing the speed of the screw can increase the plasticizing ability (some plastics that are sensitive to shear, such as PVC, PET, etc., cannot use this method).
5. Reduce the back pressure as much as possible, otherwise it will increase the plasticizing speed.
6. The hydraulic sealing nozzle is adopted, so that the mold can be plasticized when opening and closing the mold.
7. The use of pre-plasticizer design enables the screw to be plasticized during the cycle except for the injection and holding time.
8. Adopt pressure-holding installation, so that the screw can be plasticized in the pressure-holding section.
Ways 6 to 7 use parallel action, and the feeding is driven by an independent oil circuit or circuit.
Method 8 is most suitable for thick wall injection molding. In order to prevent dents, the holding time of thick wall may be lengthy.
The inverted cable before and after feeding is time-consuming, it is better to use a spring nozzle or a hydraulic sealing nozzle to replace it, so as to save the inverted cable action.
The mold is opened at the highest speed under the condition of not tearing the waste product and producing a loud mold opening sound. Some sophisticated injection molding machines have decompression equipment before mold opening, and even high-speed mold opening will not produce noise. In order to achieve accurate mold stop position under high-speed mold opening, brake valve or closed-loop control can be used.
On small injection molding machines with low ejection force, pneumatic ejection can be used, which is higher than hydraulic ejection speed. Electric ejection is faster than pneumatic ejection. The mold can be designed to be ejected by the mold opening action instead of the ejection installation on the injection molding machine. This method can only be ejected once. This is the easiest way to eject while the mold is opened.
Using independent oil circuit, gas circuit or circuit control, it can complete the function of multiple ejection while opening the mold while ejecting.
Equipped with video and computer equipment, it can quickly analyze whether all the waste products can be dropped after one ejection. The second ejection is stopped when not all of them are dropped, so 99% of the cycles in the above example are ejected only once, which saves the uniform cycle time.
The repeated ejection of some waste products can be ejected by the vibration of the injection molding machine. The thimble does not need to be fully retracted every time to shorten the time of repeated ejection. The last retreat can start at the same time as the mold clamping. Since the stroke of the thimble is shorter than that of the template, the thimble will always be fully retracted before the mold is locked.
The shortest cycle time
The shortest cycle time is composed of the time required for mold clamping, injection, pressure holding, cooling and mold opening. Feeding stops at the same time during the "cooling time" and mold opening and closing, and even when the pressure is maintained. Repeated ejection stops at the same time when the mold is opened, and the last ejection stops at the same time when the mold is closed. In this case, up to three actions can be stopped at the same time, and each action has an independent drive. It may be that all three are oil circuits (such as three oil pumps), and all three are circuits (electric injection molding machines) or a combination of oil circuits, air circuits, and circuits.
Electric injection molding machines generally have 4 servo motors, which drive injection molding, feeding, opening, closing and demolding respectively. The advantage is that the parallel operation can shorten the cycle. In fact, the hydraulic injection molding machine can also achieve this goal when using 3 independent oil circuits. Therefore, this advantage is not a patent for electric injection molding machines.
Since injection cannot be made when the mold is opened, the 4 servo motors cannot operate at the same time. The analysis revealed that the three parallel actions have reached the shortest cycle.
dry running time
Many injection molding machine specifications indicate the idle running time, but generally it is just calculated theoretical time. The acceleration and deceleration of the template are neglected. Of course, the quality of the moving mold is not calculated, which is shorter than the actual mold opening and closing time. . According to Euromap's specifications, the idle running time is the time required for the opening and closing of the template, and the opening and closing stroke is set at 0.7 times the four-column space. In the shortest cycle, as long as the opening and closing time (idle running time) and injection time are related to the design of the injection molding machine.
thin wall injection
"Thin-wall injection molding" is defined as injection molding with a wall thickness of 0.5 mm or less, or a process/wall thickness ratio of 300 or more. In order to prevent the molten plastic from solidifying before filling the cavity, thin-wall injection molding is high-speed (and high acceleration and deceleration) injection molding. The so-called high-speed injection molding means that the forward speed of the screw is above 300 mm/s. Under high-speed injection molding, the injection time is generally within 0.5s. High-speed injection is assisted by accumulators. The oil pump fills the accumulator during the "cooling time", or a small oil pump can be used to fill the accumulator during times other than injection and pressure holding. The stored high-pressure oil is released during injection, which can generally increase the injection speed by 3 times.
Thin-wall injection molding does not require pressure holding time and "cooling time", among which idle running time is the factor that determines the entire cycle time.
Blowing and demolding
If the product can be demolded by blowing, it is easy to blow while opening the mold. Normally, it delays blowing after opening the mold, hiding the demolding time in the opening time. The cost of an injection molding machine with 3 independent oil circuits is relatively high. For two parallel-action injection molding machines, one oil circuit is generally used for mold clamping installation, and another oil circuit is used for injection installation. This is based on the thinking of injection molding machine's common split clamping installation oil circuit board and injection installation oil circuit board. Hybrid oil-electric injection molding machine combines the advantages of two drives and is cleverly coordinated. The common design is electric feeding and other actions of oil pressure, such as Husky's Hyelectric injection molding machine.
Double pump injection
Dual pump design uses only one oil pump during injection. The oil circuit can be slightly changed so that the two pumps are in the same state during injection, thereby increasing the injection speed to nearly double.